Flexible printed circuit board with waterproof structure

ABSTRACT

Disclosed is a flexible printed circuit board with waterproof structure, including a flexible substrate that has a first surface on which a first metal layer is bonded. The first metal layer has an upper surface forming a covered area and at least one mounting zone and the metal layer forms on a surface thereof within the mounting zone a bonding improved waterproof structure. A first insulation layer is formed on the covered area of the upper surface of the first metal layer in such a way not to cover the mounting zone of the first metal so as to expose the mounting zone. A water resistant member is securely mounted to the mounting zone of the first metal layer. The flexible substrate also has a second surface to which a second metal layer and a second insulation layer are mounted. The second metal layer also forms a mounting zone, which is not covered by the second insulation layer so as to expose for mounting the water resistant member. The metal surface inside the mounting zone also forms bonding improved waterproof structure. The waterproof structure improves the bonding strength between the flexible printed circuit board and the water resistant member through proper arrangement and selection of shape, size, and distribution density of holes that are defined in the exposed metal surface. Alternatively, the holes can be made completely through the metal layer in order to also feature control of impedance for high frequency signals.

FIELD OF THE INVENTION

The present invention relates to a flexible printed circuit board withwaterproof structure, and in particular to a flexible printed circuitboard having a metal layer, which carries an insulation layer formedthereon and forms a mounting zone that is not covered by the insulationand is thus exposed for receiving a water resistant member mountedthereon, wherein the mounting zone comprises a bonding strengthenhancing structure to improve the bonding strength of the waterresistant member to the flexible printed circuit board, and the metallayer forms holes through a surface thereof to improve control overimpedance against high frequency signals.

BACKGROUND OF THE INVENTION

Flexible printed circuit boards are of wide applications in variouselectronic devices, such as notebook computers, personal digitalassistants, and mobile phones. A conventional flexible printed circuitboard is composed of a plurality of conductors that are respectivelycovered by an insulation jacket and are arranged to juxtapose each otherto form a flat structure. The flexible printed circuit board can beconnected through electrical connectors or soldering for transmission ofelectronic signals.

Connecting electronic devices with a flexible printed circuit board isgenerally of no concern of waterproofing. However, waterproofing maybecome a problem especially for outdoor applications or for portabledevices, such as mobile phones. Taking a mobile phone as an example, themobile phone comprises a host device portion and a display screenportion that are connected to each other by a flexible printed circuitboard that is attached to these portions through connectors orsoldering. If waterproofing between the flexible printed circuit boardand the host device portion or the display screen portion of the mobilephone is poor, then water may flow along the flexible printed circuitboard into the host device portion and the display screen portion.

To effect waterproofing or resistance against humidity, conventionally,a rubber pad is provided as a waterproof or water-resistant memberarranged between an enclosure of an electronic device and a flexibleprinted circuit board, whereby tight engagement is established betweenthe flexible printed circuit board and the electronic device enclosureto achieve the desired effect of waterproofing or resistance againsthumidity.

SUMMARY OF THE INVENTION

Although directly mounting a water resistant member on an insulationsurface layer of a flexible printed circuit board is known and is alsoeffective in achieving waterproofing or resistance against humidity.However, there is still a concern about reliability of water sealingbetween the water resistant member and the printed circuit board. One ofthe reasons is that it is hard to properly establish a tight engagementbetween the water resistant member and the surface insulation material(such as PI and insulation ink) of the printed circuit board. Althoughsurface treating agents may be applied to handle this problem, theresultant waterproofing and bonding strength are not good enough so thatany flaw occurring in the manufacturing process will lead tounacceptable bonding strength between the water resistant member and theprinted circuit board, causing undesired situations of shifting,separation, leaking of water between the water resistant member and theprinted circuit board, which eventually result in poor resistanceagainst water pressures applied thereto.

Further, when a flexible printed circuit board is employed to transmitelectronic signals of high frequency, there is always a concern aboutimpedance matching and control of signal transmission time. Since aflexible printed circuit board is of a reduced thickness, the distancebetween two surface metallic shielding layers and the signalstransmitted is very small, leading to excessively low impedance to thesignal transmission. A solution to such a problem is to form openings inlocal area or the whole area of the metallic shielding layers, whichalways lead to the desired result of impedance. It is thus of a desireto combine a structure for effecting waterproofing and bonding betweenthe water resistant member and the flexible printed circuit board and astructure for controlling high frequency impedance of the flexibleprinted circuit board, in order to improve the industrial value of aflexible printed circuit board.

Thus, an objective of the present invention is to provide a flexibleprinted circuit board with waterproof structure, which comprises aflexible substrate carrying a metal layer on which an insulation layeris formed. The metal layer forms a mounting zone that is not covered bythe insulation layer and is thus exposed. The metal layer forms holes orcorrugation on a surface thereon within the mounting zone in order toimprove the bonding strength. A water resistant member is mounted to themounting zone of the metal layer so that excellent waterproofing andbonding can be realized between the water resistant member and theflexible printed circuit board.

Another objective of the present invention is to provide awater-resistant flexible printed circuit board that features both thewaterproofing and bonding between a water resistant member and theflexible printed circuit board and the requirement of impedance for theflexible printed circuit board. With a water resistant member mounted tothe flexible printed circuit board being fit to a receptacle openingdefined in a device enclosure, excellent bonding strength is realizedbetween the water resistant member and the flexible printed circuitboard and the requirement for controlling impedance of a high frequencyflexible printed circuit board can also be achieved.

To achieve the above objective, a flexible substrate has a first surfaceon which a first metal layer is bonded. The first metal layer has anupper surface forming a covered area and at least one mounting zone. Afirst insulation layer is formed on the covered area of the uppersurface of the first metal layer in such a way not to cover the mountingzone of the first metal so as to expose the mounting zone. A waterresistant member is securely mounted to the mounting zone of the firstmetal layer. The flexible substrate also has a second surface to which asecond metal layer and a second insulation layer are selectively andsequentially mounted. The second metal layer also forms a mounting zone,which is not covered by the second insulation layer so as to expose formounting the water resistant member. In the embodiments of the presentinvention, the water resistant member mounting zone of each metal layeris provided with a bonding strength enhancing structure for securelybonding the water resistant member. The bonding strength enhancingstructure comprises at least one hole extending through the upper andlower surfaces of the metal layer or a corrugation structure that doesnot extend through the upper and lower surfaces of the metal layer. Thehole of the bonding strength enhancing structure can be of a shape ofcircle, square or rectangle, rhombus, ellipse, or triangle. In apreferred embodiment, the hole is of a size that is greater than 0.001inches. For multiple holes, it is preferable that the holes aredistributed uniformly. The holes can be formed through conventionaldevelopment/etching processes, or printing of silver paste, or bondingof pre-formed holed structure, or secondary printing of holed silverpaste layer on a surface of the metal layer to form corrugation. Theprocess is selected according to the material of the layer and the sizeand distribution density of the holes. The metal layer is made of amaterial of copper, copper with metallic surface coating, silver paste,silver foil, and aluminum foil.

The water resistant member can be made of a material showing insulatingcharacteristics, or a material showing electrical conductioncharacteristics. According to the needs of various applications, thematerial selected helps realizing the effect of insulation, electricalconductivity, or magnetic shielding. When the water resistant member ismade of an electrically conductive material, the water resistant memberserves to connect the metallic shielding layer of a flexible printedcircuit board with a metallic casing to provide improved electromagneticshielding effect.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will be apparent to those skilled in the art byreading the following description of preferred embodiments of thepresent invention, with reference to the attached drawings, in which:

FIG. 1 is a perspective view showing a flexible printed circuit boardwith water-resistant structure according to a first embodiment of thepresent invention;

FIG. 2 is a perspective view showing the present invention coupled to areceptacle opening formed in a device enclosure through a waterresistant member;

FIG. 3 is a perspective view showing the flexible printed circuit boardaccording to the present invention with the water resistant memberremoved;

FIG. 4 is a cross-sectional, in an exploded form, of the flexibleprinted circuit board according to the present invention;

FIG. 5 is a cross-sectional view taken along line 5-5 of FIG. 1;

FIG. 6 is a cross-sectional view taken along line 6-6 of FIG. 1;

FIG. 7 is a cross-sectional view, in an exploded form, showing a metallayer comprising a corrugation structure formed on a surface thereof;

FIG. 8 is a cross-sectional view showing a flexible printed circuitboard with water-resistant structure according to a second embodiment ofthe present invention; and

FIG. 9 is a cross-sectional view showing a flexible printed circuitboard with water-resistant structure according to a third embodiment ofthe present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

With reference to the drawings and in particular to FIG. 1, FIG. 1 showsa perspective view of a flexible printed circuit board withwater-resistant structure according to a first embodiment of the presentinvention, which is generally designated at 100. The flexible printedcircuit board 100 of the present invention, upon coupling to a deviceenclosure 200 (as shown in FIG. 2), inserts a water resistant member 300into a receptacle opening defined in the device enclosure 200 to protectthe device enclosure 200, which may be an enclosure of an electronicdevice, from water ingress or invasion. The flexible printed circuitboard 100 is of wide application for use in the enclosure of variouselectronic devices. For example, when used in the field of mobilephones, the flexible printed circuit board 100 can be attached to a hostdevice portion and a display screen portion through connectors 101, 102attached thereto. The flexible printed circuit board 100 may be replacedwith another type of flexible circuit board or cable, such as awell-known flexible flat cable.

FIG. 3 shows a perspective view of the flexible printed circuit board100 according to the present invention with the water resistant member300 removed. FIG. 4 shows a cross-sectional view, in an exploded form,of the flexible printed circuit board 100 according to the presentinvention. FIG. 5 shows a cross-sectional view taken along line 5-5 ofFIG. 1. FIG. 6 shows a cross-sectional view taken along line 6-6 of FIG.1.

As shown in these drawings, a flexible substrate 1 has a first surface11 on which a first metal layer 2 is formed. The first metal layer 2 hasan upper surface 21 on which a covered area 211 and at least one waterresistant member mounting zone 212 are defined. The first metal layer 2has an opposite lower surface 22 bonded to the first surface 11 of theflexible substrate 1. The first metal layer 2 may serve as anelectromagnetic shielding layer for the flexible substrate 1 and can bemade of a material of one of copper, copper with metallic surfacecoating (such as tin coating and gold coating), silver paste, silverfoil, and aluminum foil.

A first insulation layer 3 is formed on the covered area 211 of theupper surface 21 of the first metal layer 2 in such a way that the waterresistant member mounting zone 212 of the first metal layer 2 is notcovered and is thus exposed.

Similarly, the flexible substrate 1 has an opposite second surface 12 onwhich a second metal layer 2 a is formed. The second metal layer 2 a hasa lower surface to which a second insulation layer 3 a is attached. Thelower surface of the second metal layer 2 a also forms a covered area211 a and at least one water resistant member mounting zone 212 a.

A water resistant member 300 is tightly mounted to the water resistantmember mounting zone 212 of the first metal layer 2, the water resistantmember mounting zone 212 a of the second metal layer 2 a, andcircumferential portions of the flexible substrate 1. The waterresistant member 300 is made of a material showing insulatingcharacteristics, such as silicone rubber, rubber, silica gel, wherebyexcellent insulation can be achieved through the use of these materials.The water resistant member 300 may alternatively made of a materialshowing electrical conductivity, such as silicone rubber, rubber, silicagel, or resin, that contains therein electrically conductive particles(such as graphite, silver, nickel), whereby electrical conductivity andmagnetic shielding can be achieved through the use of these materials.

Preferably, the water resistant member 300, besides covering the waterresistant member mounting zone 212, 212 a, additionally covers aninterface area between the water resistant member mounting zone 212 andthe first insulation layer 3 and an interface area between the waterresistant member mounting zone 212 a and the second insulation layer 3a.

Within the water resistant member mounting zone 212, the first metallayer 2 forms at least one hole 213 (multiple holes being shown in theembodiment illustrated) extending completely through the upper surface21 and the lower surface 22 of the first metal layer 2 so as to form ahollow opening pattern in the water resistant member mounting zone 212.The arrangement of the hollow opening pattern helps controlling theimpedance and transmission time of the first metal layer 2. The holes213 are circular in the embodiment illustrated, but can be of othershapes, such as rhombus, square or rectangle, ellipse, triangle, orother suitable geometric configuration.

Besides controlling impedance and transmission time of the metal layer,the hollow opening pattern formed in the water resistant member mountingzone 212 of the first metal layer 2 is also helpful in improving thebonding strength between the water resistant member mounting zone 212 ofthe first metal layer 2 and the water resistant member 300.

Besides the holes 213 that extends through the upper surface 21 and thelower surface 22 of the first metal layer 2, the water resistant membermounting zone of the first metal layer may alternatively and/oradditionally form a corrugation structure 214 (as shown in FIG. 7) thatdoes not extend through the lower surface and the upper surface of thefirst metal layer. The corrugation structure 214 help improving thebonding strength between the water resistant member mounting zone 212 ofthe first metal layer 2 and the water resistant member 300. Thecorrugation structure 214 can be formed of patterns of circle, rhombus,square or rectangle, ellipse, triangle, or other suitable geometricconfigurations. In the embodiment described previously, a double-sidedflexible printed circuit board is taken as an example of illustration,yet the present invention is also applicable to single-sided ormulti-layered flexible printed circuit board. In the example shown inFIG. 8, a flexible substrate 1 has a first surface 11 on which a firstmetal layer 2 is formed; a first insulation layer 3 is further formed onan upper surface 21 of the first metal layer 2 within a covered area211; and a water resistant member 300 is securely mounted around aninterface area between the water resistant member mounting zone 212 ofthe first metal layer 2 and the flexible substrate 1. As compared to theprevious embodiment shown in FIG. 5, the instant embodiment comprises aflexible substrate 1 having a second surface 12 that comprises no secondmetal layer 2 a and second insulation layer 3 a.

Further, the water resistant member 300, besides being conFIG. d as aregular rectangular form, may be alternatively conFIG. d in othergeometric forms, such as a rounded water resistant member 300 a (shownin FIG. 9), in order to meet the needs of various device enclosures.

Although the present invention has been described with reference to thepreferred embodiments thereof, it is apparent to those skilled in theart that a variety of modifications and changes may be made withoutdeparting from the scope of the present invention which is intended tobe defined by the appended claims.

1. A flexible printed circuit board, comprises: a flexible substrate,which has a first surface and a second surface; a first metal layer,which has a lower surface and an upper surface, the lower surface beingbonded to the first surface of the flexible substrate, the upper surfaceof the first metal layer forming a covered area and at least onemounting zone, a bonding strength enhancing structure being formed onthe mounting zone; a first insulation layer, which is formed on thecovered area of the upper surface of the first metal layer in such a wayto expose the mounting zone of the first metal layer; and a waterresistant member, which is bonded to the bonding strength enhancingstructure of the mounting zone of the first metal layer and the secondsurface of the flexible substrate.
 2. The flexible printed circuit boardas claimed in claim 1, wherein the bonding strength enhancing structureof the first metal layer comprises at least one hole extending throughthe upper and lower surfaces of the first metal layer.
 3. The flexibleprinted circuit board as claimed in claim 2, wherein the hole is of ashape selected from a group consisting of circle, rhombus, rectangle,ellipse, and triangle.
 4. The flexible printed circuit board as claimedin claim 1, wherein the bonding strength enhancing structure of thefirst metal layer comprises a corrugation structure that does not extendthrough the upper and lower surfaces of the first metal layer.
 5. Theflexible printed circuit board as claimed in claim 1, wherein the firstmetal layer is made of a material selected from a group consisting ofcopper, copper with metallic surface coating, silver paste, silver foil,and aluminum foil.
 6. The flexible printed circuit board as claimed inclaim 1, wherein the water resistant member is made of a materialselected from a group consisting of silicone rubber containingelectrically conductive particles, rubber containing electricallyconductive particles, silica gel containing electrically conductiveparticles, and resin containing electrically conductive particles. 7.The flexible printed circuit board as claimed in claim 1, wherein thewater resistant member is made of a material selected from a groupconsisting of silicone rubber, rubber, and silica gel.
 8. The flexibleprinted circuit board as claimed in claim 1, wherein the second surfaceof the flexible substrate comprises: a second metal layer, which has alower surface and an upper surface, the upper surface being bonded tothe second surface of the flexible substrate, the lower surface of thesecond metal layer forming a covered area and at least one mountingzone, a bonding strength enhancing structure being formed on themounting zone; and a second insulation layer, which is formed on thecovered area of the lower surface of the second metal layer in such away to expose the mounting zone of the second metal layer; wherein thewater resistant member is bonded to the mounting zones of the firstmetal layer and the second metal layer.
 9. The flexible printed circuitboard as claimed in claim 8, wherein the bonding strength enhancingstructure of the second metal layer comprises at least one holeextending through the upper and lower surfaces of the second metallayer.
 10. The flexible printed circuit board as claimed in claim 9,wherein the hole is of a shape selected from a group consisting ofcircle, rhombus, rectangle, ellipse, and triangle.
 11. The flexibleprinted circuit board as claimed in claim 8, wherein the bondingstrength enhancing structure of the second metal layer comprises acorrugation structure that does not extend through the upper and lowersurfaces of the second metal layer.
 12. The flexible printed circuitboard as claimed in claim 8, wherein the second metal layer is made of amaterial selected from a group consisting of copper, copper withmetallic surface coating, silver paste, silver foil, and aluminum foil.13. The flexible printed circuit board as claimed in claim 8, whereinthe water resistant member is made of a material selected from a groupconsisting of silicone rubber containing electrically conductiveparticles, rubber containing electrically conductive particles, silicagel containing electrically conductive particles, and resin containingelectrically conductive particles.
 14. The flexible printed circuitboard as claimed in claim 8, wherein the water resistant member is madeof a material selected from a group consisting of silicone rubber,rubber, and silica gel.